1. Field of the Invention
The present invention relates to a piezoelectric ceramic and a manufacturing method therefor. More specifically, the present invention relates to a piezoelectric ceramic used for, for example, a ceramic resonator, a ceramic filter, an ultrasonic resonator and an acceleration sensor, and to a manufacturing method therefor.
2. Description of the Related Art
Hitherto, PZT compositions containing PbZrO3xe2x80x94PbTiO3 as a primary component have been widely used as compositions for piezoelectric ceramics.
Addition of metallic oxides, such as MnO2, and compound perovskite oxides, such as Pb(Nb⅔Mg,⅓)O3, to the primary component or substitution for the aforementioned oxides have been attempted to improve piezoelectric characteristics.
The PZT piezoelectric ceramics were, however, not appropriate as materials of piezoelectric elements used, in particular, in high frequency regions because the PZT piezoelectric ceramics had high dielectric constants in spite of having superior piezoelectric characteristics. Most of conventional piezoelectric ceramics primarily including the PZT piezoelectric ceramics do not have very high mechanical strength and hardness as ceramic materials. In recent years, requirements for miniaturization of electronic parts have intensified, and piezoelectric ceramic elements are also required to be compact and to exhibit sufficient characteristics. In the case in which compact elements are used in environments where shocks and vibrations are likely to be encountered, there is a problem in reliability of strength. The reason for this is that accompanying the miniaturization of elements, the effect of heterogeneity in ceramic microstructure increases so as to decrease the strength of the element as a whole.
Therefore, for the uses in high frequency regions, PT piezoelectric ceramics containing PbTiO3 as a primary component have been used. The PT piezoelectric ceramics have the merits of having low dielectric constants, of being superior in response in high frequency regions, and, in addition, of exhibiting high strength compared to the PZT piezoelectric ceramics.
Accompanying substantial miniaturization of elements, however, in accordance with strong market requirements for compact and thin layer elements, even the PT piezoelectric ceramics are insufficient in their strength.
Accordingly, objects of the present invention are to provide a piezoelectric ceramic having increased mechanical strength and which may be used for producing a piezoelectric element having sufficient strength and hardness in spite of compactness, and to provide a manufacturing method therefor.
A piezoelectric ceramic according to the present invention contains lead titanate as a primary component and about 0.1 to 5% by weight of tungsten in terms of WO3 relative to the primary component, in which about 75% or more of the crystal particles constituting the piezoelectric ceramic have particle diameters distributed in the range of about 0.2 xcexcm to 0.8 xcexcm.
A piezoelectric ceramic according to the present invention may further contain about 2% or less by weight of silicon oxide in terms of SiO2 relative to the primary component.
A manufacturing method for a piezoelectric ceramic according to the present invention includes a step of firing a piezoelectric ceramic material at a temperature of more than about 1,080xc2x0 C. and less than about 1,150xc2x0 C., in which the piezoelectric ceramic material contains lead titanate as a primary component and about 0.1 to 5% by weight of tungsten in terms of WO3 relative to the primary component.
A manufacturing method for a piezoelectric ceramic according to the present invention includes the aforementioned step of firing, in which the piezoelectric ceramic material may further contain about 2% or less by weight of silicon oxide in terms of SiO2 relative to said primary component.
It was discovered that in a piezoelectric ceramic containing lead titanate as a primary component and about 0.1 to 5% by weight of tungsten in terms of WO3 relative to the primary component, when the diameters of the crystal particles were distributed in the range of about 0.2 xcexcm to 0.8 xcexcm and were not too small, the ceramic had increased strength, and the Vickers hardness thereof was increased by limiting the diameters of crystal particles to about 0.8 xcexcm or less.
In a piezoelectric ceramic containing lead titanate as a primary component, when the piezoelectric ceramic contains less than about 0.1% by weight of tungsten in terms of WO3 relative to the primary component, an effect as a sintering promoter cannot be seen, and the piezoelectric ceramic cannot be sintered at a low temperature. When the content exceeds about 5% by weight, the three-point flexural strength and Vickers hardness are decreased. Therefore, neither of the aforementioned cases is preferable.
It was discovered that a piezoelectric ceramic containing lead titanate as a primary component and about 0.1 to 5% by weight of tungsten in terms of WO3 relative to the primary component, in which about 75% or more of crystal particles constituting the piezoelectric ceramic had particle diameters distributed in the range of about 0.2 xcexcm to 0.8 xcexcm, could be produced by firing a piezoelectric ceramic material containing lead titanate as a primary component and about 0.1 to 5% by weight of tungsten in terms of WO3 relative to the primary component at a temperature of more than about 1,080xc2x0 C. and less than about 1,150xc2x0 C.. Herein, when the firing temperature is 1,080xc2x0 C. or less, sintering does not sufficiently proceed, and when the firing temperature is 1,150xc2x0 C. or more, growth of particles cannot be sufficiently suppressed. Therefore, neither of the aforementioned cases is preferable.
Furthermore, in the piezoelectric ceramic containing lead titanate as a primary component according to the present invention and in the manufacturing method therefor, by further containing about 2% or less by weight of silicon oxide in terms of SiO2 relative to the primary component, the distribution of the diameters of crystal particles can be controlled.
The aforementioned objects, further objects, features, and advantages of the present invention will be apparent from the following detailed explanations of embodiments according to the present invention with reference to the drawings.